The present invention relates to a reduction cell pot used in the production of aluminum via fused salt electrolysis. The cell comprises a steel shell, a thermally insulating layer and an inner lining comprised mainly of carbon.
In the fused salt electrolytic production of aluminum from aluminum oxide, the latter is dissolved in a fluoride melt comprised for the most part of cryolite. The cathodically precipitated aluminum collects on the carbon floor of the cell underneath the fluoride melt, the surface of the pool of liquid aluminum collected being the cathode. Dipping into the melt from above are anodes which in conventional processes are made of amorphous carbon. At the carbon anodes, as a result of the electrolytic decomposition of the aluminum oxide, oxygen forms and combines with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place in the temperature range of approximately 940.degree.-970.degree. C.
The electrical energy consumed in the electrolytic process can be divided into two main categories:
(1) Production or reduction energy PA1 (2) energy losses.
The productive part of the energy consumed is required to reduce the cations to metallic aluminum. This productive part of the energy can, therefore, not be reduced.
The unproductive energy losses however can be divided into different types of losses which appear as heat losses to the surroundings. These heat losses can be regulated and must be reduced to a minimum.
This can be achieved by using the most suitable material for conductor bars, whereby the voltage drop and thus the energy losses in the electrical circuit can be reduced to a minimum.
The heat produced in the electrolytic process always flows to the colder parts of the pot and from there to the surroundings thereby producing a loss of energy from the actual production process.
In order to prevent this loss through the pot, or at least to reduce it to a low level, the practice of having a thermally insulating layer in the outer steel sheel has been common in the art. For the insulating layer, shaped pieces made of diatomaceous earth or moler brick are normally employed. New moler bricks have excellent insulation properties. However, the moler bricks are very sensitive to components of the molten bath which penetrate the carbon lining. For the foregoing reason the innermost insulating layer is often made of less sensitive but poorer insulating firebrick. Bricks can be easily stacked one upon the other which allows the side walls and the horizontal floor of the pot to be insulated without problem.
It has also been proposed, for example in the U.S. Pat. Nos. 4,001,104 and 4,052,288, to employ granulated insulating material such as alumina instead of the pre-shaped bricks. Granular material, however, is generally used only for horizontal layers that is for insulating the floor of the pot. To insulate the side walls of the pot it is as customary to use insulating bricks which can be built up one upon the other.
Alumina is inert with respect to bath components penetrating the carbon lining however the thermal insulation capacity of a pot floor lined with alumina is relatively low.
When a pot has to be replaced or renewed, the lining is broken off and in most cases has to be thrown away. If the insulating material is alumina, it is possible to recycle the aluminum oxide from the floor insulation, provided the appropriate equipment is available in the smelter in question. Quite generally it can be said that such a recycling facility is a decisive factor as far as the use of alumina for pot floor insulation material is concerned.
The use of moler bricks and alumina as insulating material represents a considerable cost factor for an aluminum smelter as both materials must be described as expensive. Furthermore, molar bricks have the disadvantage that they continuously lose their good thermal insulation property as soon as they are impregnated with bath components leaking through the carbon lining. Thus, before one third of its useful service life is passed, service life being approximately five years, a pot can lose the greater part of its thermal insulation capacity. In other words, the electrolytic cell runs for two to three years without any effective thermal insulation and therefore wastefully releases a considerable amount of energy to the surroundings over that extended period.
It is therefore the principal object of the present invention to provide the pot of an electrolytic cell with a layer of thermal insulation which exhibits good insulating properties over the whole of the lifetime of the pot and can be manufactured with much lower investment costs than the insulation used up to date.